Process for separating aqueous formaldehyde mixtures

ABSTRACT

Process for the separation of water from mixtures of water and formaldehyde, by contacting the mixtures against an organic polymeric membrane, and withdrawing at the other side of the membrane a mixture having a higher concentration of water.

This is a division of application Ser. No. 191,097 filed Oct. 20, 1971,now U.S. Pat. No. 3,950,247, which is a Continuation of Application Ser.No. 46,801 filed June 16, 1970, now abandoned.

SEPARATION PROCEDURE

The present invention relates to the separation of components, one ofwhich is water, such as formaldehyde and water, in order to obtain amore highly concentrated solution by removing at least a portion of thewater from the feed solution. In general the feed solutions are composedor organic compounds soluble in water, or inorganic compounds soluble inwater.

The feed solutions may also contain additional components, e.g.,methanol with formaldehyde-water, or sodium chloride with hydrochloricacid-water, or butanol in ethanol-water systems.

Essentially the present process comprises contacting the feed mixtureagainst one side of a membrane, and withdrawing at the second side amixture having a higher concentration of water than the aforesaid feedmixture. It is also essential that the mixture at the second side bemaintained at a lower chemical potential than the feed side. It is alsoessential that the product be withdrawn at the second side in the vaporstate. In the commercial utilization of the present process multistateoperation may also be feasible since this permits the operation ofindividual stages at various concentrations and temperatures in order toachieve the optimum driving forces for the process.

For each individual stage the effectiveness of the separation is shownby the separation factor (S.F.).

The separation factor (S.F.) is defined as the ratio of theconcentrations of the two substances A and B to be separated dividedinto the ratio of the concentrations of the corresponding substances inthe permeate ##EQU1## WHERE C_(A) and c_(B) are the concentrations ofwater and formaldehyde (HNO₃, HCl or ethanol), respectively.

In a preferred embodiment of the invention the first or feed side of themembrane is under a positive pressure, while the second side is under anegative pressure, relative to atmospheric pressure. Still morepreferably the second side is maintained at a pressure differentialwhich is greater than 0.01 atmosphere, or preferably with a differentialof from 0.1 to 0.5 atmosphere. Another preferred mode of operation iswith the second side maintained at a vacuum of from 0.2 mm. to 759 mm.Hg.

At least one of the components to be separated are characterized bypronounced hydrogen bonding. Thus water is the major component orsolvent although other compounds such as alcohols, esters and organicacids may be the solvent phase in the liquid or vapor feed.

The additional component which is generally to be concentrated bypreferentially removing the water or other solvent from azeotropic andnon-azeotropic systems include as typical:

    ______________________________________                                        benzene          aniline                                                      butanol          2-ethylhexanol                                               acetic acid      hydrogen peroxide                                            formic acid      hydrazine                                                    picoline         nitromethane                                                 methyl fumarate  acrolein                                                     cyclohexanol     propionaldehyde                                              triethylamine    1,3-dioxolane                                                triethanolamine  methacrylonitrile                                            hydrofluoric acid                                                                              crotonaldehyde                                               isopropanol      vinyl acetate                                                n-propanol       butyronitrile                                                hydrocyanic acid ethyl vinyl ether                                            carbon tetrachloride                                                                           p-dioxane                                                    carbon disulfide methyl propionate                                            chloroform       pyridine                                                     trichloroethylene                                                                              ethyl acrylate                                               acetonitrile     vinyl propionate                                             chloroethanol    ethyl propionate                                             acrylonitrile    picoline                                                     allyl alcohol    cyclohexanone                                                propionic acid   butyl vinyl ether                                            methyl acetae    cyclohexylamine                                              methyl acrylate  hexyl alcohol                                                butanone         hexylamine                                                   butyraldehyde    butyl acetate                                                isobutyraldehyde isooctyl alcohol                                             butyric acid     dibutylamine                                                 ethyl acetate    decyl alcohols                                               isopropyl ether  propionitrile                                                ethyl ether      propyl acetate                                               butyl ether      amyl alcohol                                                 furaldehyde      amyl acetate                                                 furfuryl alcohol                                                              furfurylamine                                                                 methyl methacrylate                                                           pentanone                                                                     ethyl carbonate                                                               piperidine                                                                    phenol                                                                        ______________________________________                                    

The term "chemical potential" is employed herein as described by Olaf A.Hougen & K. M. Watson ("Chemical Process Principles, Part II", JohnWiley, N.Y., 1947). It is related to the escaping tendency of asubstance from any particular phase. For an ideal vapor or gas thisescaping tendency is equal to the partial pressure so that it variesgreatly with changes in the total pressure. For a liquid the change inescaping tendency as a function of total pressure is small. The escapingtendency always depends upon the temperature and concentration. In theinvention described herein, the feed substance is usually a liquidsolution, and the other side of the membrane is maintained such that avapor phase exists. A vapor feed is especially advantageous when themixture to be separated is available in that form from an industrialprocess or when heat economies are to be effected in a multistageprocess.

The feed side may be at pressures less than atmospheric, or greater thanatmospheric, and also at pressures over and above the vapor pressure ofthe liquid components (e.g. the flowing gaseous phase when a nitrogen,helium or other gaseous atmosphere is employed). The collection orpermeate vapor side of the membrane may be at less than or greater than,atmospheric pressure. The total pressure on the feed side is preferablybetween 0 psi absolute and 5000 psig, preferably between 0 psi absoluteand 1000 psig. The vapor or collection side is maintained at a totalpressure of 0 psi abs. to 1000 psig, preferably between 0 psia and 500psig. The conditions are always such as to maintain a higher chemicalpotential on the feed side than on the collection or vapor side, asdefined above.

The liquid-vapor permeation as described above takes place through apermeable membrane. This membrane may be a simple disk or sheet of themembrane substance, which is suitably mounted in a duct or pipe, ormounted in plate and frame filter presses. However other forms ofmembrane may also be employed such as hollow tubes and fibers throughwhich or around which the feed is supplied or is recirculated, with theproduct being removed at the other surface of the tubes as a vapor.Various other shapes and sizes are readily adaptable to commercialinstallations.

The process of the invention accomplishes the separation of thecomponents of mixtures one of whose components is water, by the removalof the water component through a permeable membrane with the water in ahigher concentration than in the feed being removed from one side of themembrane as a vapor, and with the imposition of a state of lowerchemical potential on such collection side of the membrane. Thus aformaldehyde-water solution may be applied at atmospheric pressure toone side of a flat diaphragm of polyvinyl butyral, or other polymer, toaccomplish a removal of at least a part of the water, leaving a morehighly concentrated formaldehyde solution at the feed side of themembrane or diaphragm. Another advantage of the present process is thatit can produce less formic acid during the concentration of formaldehydesolution than does the conventional distillation process. In onepreferred embodiment of the invention the membrane is a syntheticorganic polymeric substance characterized by the presence of anionicgroups within the polymer. The anionic groups may also be active acidicgroups within the polymer, for example sulfonic acid groups.

As an example of a state of lower chemical potential on the collectionor downstream side of the membrane, a vacuum may be maintained on thecollection side, e.g., from 0.1 mm. to the vapor pressure of water inthe feed solution at the membrane, at the respective temperature, aslong as the vapor phase is present on the downstream side. A preferredrange of vacuum is from 0.2 mm. to 759 mm. Hg.

In the formaldehyde-water system referred to above, the waterselectively passes through the permeable membrane with the water-richcomposition being rapidly removed as a vapor from the collection side ofthe membrane.

In contrast to the present invention, the employment of a liquid phaseon each side of the membrane is seeking to accomplish significantseparation is impractical because the applied pressure, has been foundto be prohibitively high, e.g., up to 1000 atmospheres being necessarybecause of osmotic pressure. The liquid-liquid permeation is largely anequilibrium phenomenon, unless the osmotic forces are overcome, while incontrast, the liquid-vapor or vapor-vapor permeation of the presentinvention is a rate process, even at moderate conditions, in which thevapor is removed as soon as it reaches the collection surface of themembrane. Consequently it is found that a comparison of theeffectiveness between liquid-liquid permeation, and the presentliquid-vapor or vapor-vapor permeation for separation processes involvesa significant superiority for the present method.

The present invention is of particular utility, with liquid feed stocks,including normally liquifiable feeds such as propylene, and is lessuseful for the separation of fixed gases such as hydrogen, nitrogen,helium and methane. These fixed gases have appreciably lower solubilityin polymers than do liquids and saturated vapors, with the resultantlower rates of permeation.

It has been found that very effective membranes are composed of organicpolymers having active anionic groups derived from strong acids.Preferred anionic or acidic moieties or end groups include sulfonic(--SO₃ --), phosphonic (--PO₃ ⁻⁻), phosphinic (--HO₂ P⁻), arsenic,(--AsO₃ ⁻), for example, and selenonic (--SeO₃ ⁻), and telluric (--TeO₃⁻) in their various valence forms. Suitable organic anionic groupsinclude:

    ______________________________________                                        Methacrylic acid                                                                             ##STR1##                                                       Maleic acid (and its isomer fumaric acid)                                                    ##STR2##                                                       Acrylic acid                                                                                 ##STR3##                                                       Vinylacetic acid                                                                             ##STR4##                                                       Vinylpropionic acid                                                                          ##STR5##                                                       Aconitic acid                                                                                ##STR6##                                                       Sorbic acid                                                                                  ##STR7##                                                       Vinylbenzoic acid                                                                            ##STR8##                                                       Itaconic acid                                                                                ##STR9##                                                       But=2,3,4-tricarboxylic acid-ene-1                                                           ##STR10##                                                      ______________________________________                                    

The membranes containing anionic groups may contain a formal negativecharge such that a counterion (cation) may be present. This cation maybe monovalent or multivalent, e.g., H⁺, Na⁺, K⁺, Mg⁺⁺, Al⁺⁺⁺ or R₄ N⁺. Awide variety of cations are useful even though the exact value of theseparation factor is dependent on the specific cation used. While notbound by theory we believe that the effect of the counterior istwo-fold: (1) its effect on the membrane morphology and structure and(2) its effect on the ability of the anion to bond water and the bindingof water by the cation itself. When a particular cation is preferred,e.g., to maintain the pH level, this cation can be preserved in themembrane by the addition of small quantities of salts in the feed, (e.g.NaCl). In general the polymers can be used as the acid form or as thevarious salts as well as other derivatives such as the esters.

Carriers which may be used as membranes either as polymers, copolymersor mixtures as well as in modified form to provide anionic groups as endgroups or as pendant groups along the polymer chain include:polyacrylate esters, polymethacrylate esters, polyvinyl chloride,polyvinylidene chloride, after-chlorinated polyvinyl chloride,poly-3,3-bis(chloromethyl)oxetane, polyvinyl fluoridepolyvinylidenefluoride, polychlorotrifluoroethylene,polytetrafluoroethylene, poly(vinyl trifluoroacetate), poly(methyl vinylcarbinol), poly(vinyl alcohol), poly(vinyl hydrogen phthalate),poly(vinyl acetate), poly(vinyl chloroacetate), poly(vinyldichloroacetate), poly(vinyl trichloroacetate), poly(vinylcyclohexane),poly(acenaphthalene), poly(vinyltoluene), poly(vinyl naphthalene),poly(alpha methylstyrene), polystyrene and substituted styrenes (chloro,nitro, alkyl with 1 to 20 carbons and alkoxy with 1 to 20 carbons),polyvinyl sulfide, aromatic polysulfones, poly(ethyl vinyl sulfone),polyethers (e.g. polypropylene oxide), polyacetals, polyketones,polyesters (aromatic and aliphatic), polyvinyl butyral, polyvinylformal, poly(alkyl vinyl ethers), poly(aryl vinyl ethers), poly(allyllicresins), cellulose butyrate, cellulose propionate, cellulose ethers(with 1 to 20 carbon atoms in the ether group), silicon-containingresins, epoxy resins, polyphenylene oxide, polycarbonates, polyolefinssuch as ethylene, propylene, butene, isobutylene, 4-methylpentene-1, andcopolymers, polydienes such as butadiene and isoprene, furane resins,phenolic resins, cresyllic resins.

Preferred members as carriers include: polyvinyl chloride,polyvinylidene chloride, after-chlorinated polyvinyl chloride,poly-3,3-bis(chloromethyl)oxetane, polyvinyl fluoride, polyvinylidenefluoride, polychlorotrifluoroethylene, polytetrafluoroethylene,poly(vinyl trifluoroacetate), poly(acenaphthalene), poly(vinylnaphthalene), aromatic polysulfones, polyketones, polyester (aromatic, 6to 20 carbon atoms and aliphatic, 2 to 20 carbon atoms, poly(allylicresins), silicon-containing resins, epoxy resins, polyphenylene oxideand polycarbonates.

Many polymers are also useful by themselves (e.g. without specificallybuilding in groups) since they contain their own groups which aresufficiently anionic in character. Examples of such polymers include:polyvinyl chloride, polyvinylidene chloride, after-chlorinated polyvinylchloride, poly-3,3-bis(chloromethyl) oxetane, polyvinyl fluoride,polyvinylidene fluoride, polychlorotrifluoroethylene,polytetrafluoroethylene, poly(vinyl trifluoroacetate), aromaticpolysulfones, poly(ethyl vinyl sulfone), polyethers (e.g. polypropyleneoxide), polyacetals, polyketones, polyester (aromatic, 6 to 20 carbonatoms and aliphatic, 2 to 20 carbon atoms), polyvinyl butyral, polyvinylformal, poly(alkyl vinyl ethers) poly(aryl vinyl ethers), polyphenyleneoxide, and polycarbonates. The effective groups can be contained orintroduced into polymers in a variety of ways, for example, grafting,formation of block polymers or via Diels-Alder reactions. The polymersmay be rubbery or stiff. Single or multilayered films can be used.

It has also been found that improved permeation can occur if thepolymeric membrane is heat treated. In general heating the film dry orwet at a temperature of from 50° to 400° C, (if dry, preferably in anitrogen atmosphere) improves the separative properties.

Another important control over the separation capacity of the membranesis exercised by the method used to form and solidify the membrane (e.g.casting from a melt into controlled atmospheres or from solution intobaths at various concentrations and temperatures).

The polymeric substances preferably have the anions of acids present inthe polymer chain. The preferred anions are those of strong acids, asindicated by the pK values of the acid moiety, e.g., H₃ PO₄ ⃡H⁺ + H₂ PO₄⁻ (pK=2.12). In a preferred embodiment of the invention the pK value isfrom 0.1 to 5 for at least one of the dissociating groups, and stillmore preferably 0.1 to 3.

The anionic groups may be incorporated into the polymer bycopolymerization, e.g., maleic anhydride copolymerized with methyl vinylether, or the anionic groups may result from the use ofanionic-producing polymerization catalysts, e.g., potassiumpersulfate/sodium bisulfite employed with acrylonitrile, or asulfonate-containing organic peroxide or the groups may be incorporatedby reaction on the finished polymer, e.g., the reaction ofchlorosulfonic acid with a copolymer of styrene and acrylonitrile. Thusthe anionic groups may be pendant along the molecular chain or may bepresent as end groups.

The following table lists the pK values of common acids for illustrativepurpose. These materials can be considered to be model compounds for theacid groups in the various polymers.

    ______________________________________                                        pK of Common Acids                                                            in Aqueous Solutions (25° )                                            Acid Moiety       Step       pK                                               ______________________________________                                        Arsenic acid      1          2.25                                                               2          6.77                                                               3          11.60                                            Fumaric acid      1          3.03                                                               2          4.44                                             Tricarboxy-1,3,4-but                                                                            1          3.18                                             1-ene             2          4.52                                                               3          5.99                                             Itaconic acid     1          3.85                                                               2          5.45                                             Carbonic acid     1          6.37                                                               2          10.25                                            Methacrylic acid  1          3.66                                             Acrylic acid      1          4.25                                             Phosphoric acid   1          2.12                                                               2          7.21                                                               3          12.67                                            Methyl phosphonic acid                                                                          1          1.23                                                               2          2.79                                             Selenic acid      2          1.92                                             Sulfuric acid     2          1.92                                             Maleic acid (derived                                                                            1          1.83                                             from maleic anhydride                                                                           2          6.07                                             Benzenesulfonic acid                                                                            1          0.70                                             ______________________________________                                    

Preferred components include: phosphonic acids, phosphinic acids,tricarboxy-2,3,4-but-1-ene acid, the organic acids from selenium,methacrylic acid, acrylic acid, sulfonic acids, itaconic acid, anhydridewhich yields maleic acid, and fumaric acid.

The following examples illustrate specific embodiments of the presentinvention.

EXAMPLE 1

This example shows the concentration, i.e. water removal fromformaldehyde, using membranes of a copolymer of styrene and acrylicacid, made according to the following general procedure.

In this procedure one mole of styrene and 0.07 moles of acrylic acid areadded to dimethylformamide (one liter) in a reaction vessel having areflux condenser. The flask is maintained at 50° C, under nitrogenpressure, while the catalyst is added (catalyst isazobisisobutyronitrile). The reaction is stopped by the addition of a 1%solution of tertiary butyl catechol. The mixture is cooled and added to10 volumes of methanol, filtered, washed with water and methanol anddried at 40° C under vacuum.

The membranes are prepared by casting films from a 5% dimethyl formamidesolution of the polymer as described below, containing varying amountsof the acid group, on a glass plate heated on a steam bath. After dryingone hour on the steam bath, the membrane is conditioned by immersion inwater followed by treatment with the permeating solution in the cell atleast a day prior to measurement. The thickness of the membrane in watervaries from 1.0 to 1.2 mils.

The present example is directed to the concentration of formaldehyde viathe permeable membrane. Formaldehyde concentration is determined by thequantity of NaOH liberated by reaction of HCHO with sodium sulfite (inexcess) to form the formaldehydebisulfite addition product according tothe equation:

    HCHO (aq.) + Na.sub.2 SO.sub.3 + H.sub.2 O → NaOH + CH.sub.2 (NaSO.sub.3)OH

the percentage of HCHO is calculated by the equation: ##EQU2##

The separation factor (S.F.) is defined as the ratio of theconcentrations of the two substances A and B to be separated dividedinto the ratio of the concentrations of the corresponding substances inthe permeate: ##EQU3## where c_(A) and c_(B) are the concentrations ofwater and formaldehyde (HNO₃, HCl or ethanol), respectively.

A formalin solution containing 39.95% HCHO, 53% H₂ O, 0.05% formic acidand 10% CH₃ OH is concentrated into a slurry containing 62% HCHO at roomtemperature. The separation factor is above 5.

The formic acid is also selectively permeated (relative to formaldehyde)during this operation.

EXAMPLE 2 to 5

Poly(vinyl butyral), which can be considered as a condensation productof polyvinyl alcohol and butyraldehyde, contains an acetal linkage. Theacetal oxygen is hydrophilic and weakly anionic. The membrane preparedfrom poly(vinyl butyral) is also selective, giving a separation factorof 14-18. The following table compares the separation factors obtainedwith various membranes for permeation of formaldehyde solutions.

    ______________________________________                                        Comparison of S.F.'s obtained for Formaldehyde                                Solutions with Various Membranes by Permeation                                Example   Membrane            S.F.                                            ______________________________________                                        2         Cellulose acetae    2-8                                             3         Silicone polycarbonate                                                                            3.0                                                       (Tradename M-213 by CE)                                             4         Styrene/methacrylic acid                                                                          > 3                                             5         Poly(vinyl butyral) 14-18                                           ______________________________________                                    

EXAMPLE 6

It has been found that the membranes prepared from copolymers ofethylene and acrylic acid (in which the acrylic acid moiety is pendentto the chain) which are already characterized by high selectivity, maybe further improved. The performance is remarkably improved by replacingthe weaker anion (namely the acrylic acid with a pK=5) with strongeranionic groups (the sulfonic acid group with a pK=1). Acrylic acid has arelatively low degree of ionization when compared to the sulfonic acidgroup which is virtually completely ionized.

The sulfonic acid groups are introduced into the polyethylene byallowing chlorosulfonic acid to react with polyethylene film at 25° Cfor 1 to 2 hours.

The data below summarizes the results of concentratingformaldehyde-water using sulfonated polyethylene films.

    ______________________________________                                        Permeation of 37% Formalin Solutions                                          Through Sulfonated Polyethylene Films                                         Relative Degree of Sulfonation                                                                     Separation Factor                                        ______________________________________                                        low                  17                                                       high                 12                                                       ______________________________________                                    

The results obtained on the permeation of formaldehyde solutions withthe sulfonated polyethylene membranes are given in Example 6. While theS.F. changes in a relatively minor way, the permeation rate (not shown)increases with the concentration of the acid group as well as with thetemperature.

EXAMPLE 7

The following example illustrates the separation method of thisinvention in which the separation a three-component system-water,methanol and formaldehyde is involved. The separation factors withrespect to formaldehdye and methanol are given as follows:

    ______________________________________                                        Permeation of water, methanol and formaldehyde with                           an Ethylene Membrane containing Sulfonic Acid Groups                          ______________________________________                                        Temp. of                                                                              % HCHO in  % HCHO in  S.F. of water with                              Permeation                                                                            Permeant   Permeate   respect to HCHO                                 ______________________________________                                        40      57.28      below 10   above 5                                         40      38.81      below 7    above 10                                        Temp. of                                                                              %CH.sub.3 OH in                                                                          %CH.sub.3 OH in                                                                          S.F. of water with                              Permeation                                                                            Permeant   Permeate   respect to CH.sub.3 OH                          ______________________________________                                        40      31.17      below 8    greater than 3                                  40      11.20      below 2    greater than 2                                  ______________________________________                                    

The different separation factors obtained for HCHO and CH₃ OH indicateclearly that the rate of permeation of H₂ O, HCHO and CH₃ OH can bearranged in the following order

    H.sub.2 O > CH.sub.3 OH > HCHO

consequently the membrane can be used for the separation of CH₃ OH fromother organic materials.

The sulfonate groups are incorporated into the membrane either byreaction of the polyethylene with chlorosulfonic acid or by admixingpolyethylene and polyethylene containing sulfonic acid groups to givethe desired concentration. Apart from the difference in the rate ofpermeation, the separation factors obtained with these membranes areessentially the same.

EXAMPLE 8

This example illustrates the fact that the copolymer containing thesulfonate groups can be prepared by the copolymerization of styrene andsodium allylsulfonate (SAS) instead of SSS.

The results obtained are given in the following table

    ______________________________________                                        Permeation of Formaldehyde Solutions with Membrane                            Prepared from a copolymer of Styrene and SAS                                  containing 2 Mole % of SAS (25° C)                                     ______________________________________                                        Permeation                                                                              HCHO in                                                             Time, hrs.                                                                              Permeant   Permeate   S.F.                                          ______________________________________                                        2.25      38.25      below 7    above 10                                      ______________________________________                                    

The strong anionic group which is incorporated within the polymer doesnot have to be derived from a second monomer. It can be incorporated asa fragment from the particular catalyst used in end groups. Only a smallfraction of the end groups need to contain anions.

EXAMPLE 9

Polystyrene is produced by the procedure outlined on page 220 of"Preparation Methods of Polymer Chemistry" by Sorenson & Campbell,Interscience, 2nd Ed., 1968 and used to permeate formalin at 25°. Theseparation factor is above 6. When polystyrene is obtained by the bulkpolymerization using an azobisisobutyro-nitrile catalyst, the separationfactor is less than 3. This difference is due to the sulfonate endgroups in the former material.

EXAMPLE 10

This example shows the concentration of formaldehyde at hightemperatures and at high and low concentration in the feed cell.

Polyethylene terephthalate is used at 70° C with liquid on one side ofthe membrane (which contains 14% formaldehyde and 84% water) and apressure of less than 0.1 mm on the collecting side of the membrane. Thepermeate contains less than 0.2% formaldehyde giving a separation factorof greater than 30.

In another experiment the polyester is held in contact with a liquidsolution of 25% formaldehyde in water at 70° C. The vapor side is at apressure of less than 0.1 mm Hg. The permeate contains 1.0%formaldehyde, giving a separation factor above 20. To accomplish aconcentration by reverse osmosis (liquid phase to liquid phase) underthese conditions would require pressures well in excess of 1000 psi.With reverse osmosis the separation factor would be less than 2.

EXAMPLE 11

The following table shows other data on permeation of formaldehyde-watersolutions through various membranes.

    ______________________________________                                        PERMEATION OF FORMALIN AT 25° C                                        (vapor side less than 0.1 mm Hp                                               and feed side at one atmosphere)                                              Polymer                 Separation Factor                                     ______________________________________                                        1.  Copolymer styrene and methacrylic                                                                     5                                                     acid                                                                      2.  Polyethylene sulfonic acid (by reac-                                                                  17                                                    tion of ClSO.sub.3 H with polyethylene                                    3.  Copolymer of styrene and isoprene                                                                     12                                                4.  Copolymer of ethylene and vinyl acetate                                                               3                                                 5.  Polyvinyl chloride      80                                                ______________________________________                                        __________________________________________________________________________    Permeation of formalin at 25° C thru physical blends of polymers       (vapor side less than 0.1 mm Hg)                                              Component A                                                                   Type                                                                          Polymer % by Wt.                                                                             Component B      Separation Factor                             __________________________________________________________________________    Polystyrene                                                                           80     copolymer of maleic anhydride                                                                  > 20                                                         and methyl vinyl ether                                         "       80     copolymer of styrene and                                                                       > 10                                                         itaconic acid                                                  "       80     copolymer of vinyl cyclohexane                                                                 > 10                                          "       80     copolymer of styrene and                                                                       > 10                                                         maleic anhydride                                               __________________________________________________________________________

EXAMPLE 12

While not bound by theory, the mode of operation of this inventionappears to depend on the ability of water to be bound by hydrogen bondsand to form hydrogen bonds. Thus, the sulfonic acid group has beenreported to "absorb" as many as 15 molecules of water. Other hydrogenbonding materials include oxygen, sulfur, phosphorus, selenium, andtellurium groups.

A 37% commercial formalin solution is kept in contact with a film ofAraldite 488N (an epoxy resin, Ciba Inc.) cast on filter paper, and avacuum is maintained on the other side of the film. The permeatecontains less than 7% formaldehyde and the separation factor is greaterthan 8. Similarly, other oxygen rich materials are useful, e.g.,polyesters of ethylene glycol and bis-hydroxymethylcyclohexane.

EXAMPLE 13

A film is produced by casting a solution of a copolymer of poly(vinylchloride and vinyl alcohol) on glass and maintaining the wet film undervacuum for 8 hours at 35° C. When one side of the film is exposed to 14%formalin and the other side to a vacuum the permeate contains less than0.5% formaldehyde and the separation factor is greater than 30.

EXAMPLE 14

This example shows the concentration of nitric acid solution by removalof some of the water from dilute solution. The results on permeation ofnitric acid with a vinylidene fluoride membrane containing sulfonic acidgroups are given as follows at 25° C:

    ______________________________________                                                % HNO.sub.3 by                                                                             % HNO.sub.3 by                                           Example wt.-permeant wt.-permeate S.F.                                        ______________________________________                                        14      32.7         less than 10 above 5                                     ______________________________________                                    

EXAMPLE 15

    ______________________________________                                        Permeation of Nitric Acid Solutions (32% by weight)                           with Membranes Prepared from Polyblends of                                    Polyethylene and Sulfonated Polyethylene                                      at different concentration levels (25° C)                              ______________________________________                                                   Conc. of Sulfonic                                                             Acid Groups    S.F.                                                ______________________________________                                        A.   Polyblends  low              above 10                                                     high             above 5                                     B.   Copolymers  low              above 10                                                     high             above 10                                    ______________________________________                                    

EXAMPLE 16

Certain fluorinated polymers such as KEL-F, ethylenetetrafluoroethylenecopolymer, Fluorel and Viton, etc., are stable toward conc. HNO₃ even atmoderately high temperatures and the membrane prepared from thesepolymers are highly selective. Both Fluorel and Viton are copolymers ofvinylidene fluoride and hexfluoropropene. Fluorel, an elastomer, is thetradename of the MMM Company and Viton is the tradename of the duPontCompany.

    ______________________________________                                        Permeation of HNO.sub.3 solutions with a Membrane                             prepared from a copolymer of ethylene and                                     tetrafluoroethylene at a molar ratio of 56:15,                                respectively                                                                  Temp.,  wt. % HNO.sub.3 in                                                                           wt. % HNO.sub.3 in                                     ° C                                                                            Permeant       Permeate       S.F.                                    ______________________________________                                        25      30.7           0.56           78                                      25      50.7           2.12           48                                      25      69.5           20.8           8.7                                     50      50.7           7              15                                      75      50.7           12             8                                       65      50.7           8              12                                      58      50.7           9              10                                      ______________________________________                                    

The rate (not shown) increases with temperature. It is expected that therate is further increased by incorporating the sulfonic acid groups inthe fluorinated polymers. Such a membrane (XR Membrane) gives highpermeation rates, and selectively as well as high chemical stability.

XR Membrane is a fluoropolymer manufactured by the duPont Company (AIChEMaterials Engineering Conference, Atlanta, Ga., Feb. 15-18, 1970). TheXR Membrane contains varying amounts of sulfonic acid groups expressedby the weight of the polymer containing 1 equivalent of the sulfonicacid group. The equivalent polymers weight ranges from 900 to 2000.

The fluorine containing polymers by themselves provide weak anionicgroups, although sulfonic acid and other anions may also be present.

The said copolymer is composed of a backbone which is stable toward thepermeant and contains groups which are hydrophilic and highly permeableto water. Fully fluorinated copolymers containing sulfonate or sulfonicacid groups are preferred.

The said polymer, with or without acid groups, is hydrophilic (e.g.polyvinyl butyral) which is preferentially permeable to water.

EXAMPLE 17

A copolymer of vinyl chloride and vinyl alcohol is used to separate a20% by weight solution of hydrochloric acid in water at 25° C. Thepressure on the permeate side is below 1 mm of mercury. The separationfactor is above 5.

EXAMPLE 18

Liquid-vapor permeation with the polyvinyl alcohol copolymer membrane isalso carried out for dewatering of ethanol solutions. The separationfactors at 25° C are above 5.

The separation of water from azeotropic mixtures is also found to bereadily accomplished with a good separation factor for liquids selectedfrom the class consisting of acrolein, methacrylonitrile, acrylonitrile,vinyl acetate, pyridine, ethyl acrylate, acetonitrile, hydrocyanic acid,isopropanol, normal propanol, cyclohexanol, formic acid, acetic acid,butanol, acrylonitrile, propionic acid, methyl acetate, methyl acrylate,butyraldehyde, isobutyraldehyde, ethyl acetate, ethyl ether, furfurylalcohol, methyl methacrylate, phenol aniline, 2-ethylhexanol,cyclohexylamine, butyl acrylate, isooctyl alcohol, propionitrile, amylalcohol, amyl acetate and ethyl alcohol.

EXAMPLE 19

The examples below shows that the pressure on both sides of the membranemay be above or below atmospheric pressure.

A 70% by weight aqueous solution of acetaldehyde is allowed to permeatethru a polyvinyl fluoride membrane containing one mole percent ofsulfonic acid groups as sodium styrene sulfonate. The solution is heldat 115° C (above 2000 mm. of mercury pressure) and the other side of themembrane is held at 105° C (about 900 mm. of mercury pressure). Theseparation factor as previously defined is above 5.

The same feed solution is held at 115° C and the downstream side of themembrane is held at 85° C (about 450 mm. of mercury pressure). Theseparation factor is above 5.

EXAMPLE 20

A 3% by weight solution of maltol in water is allowed to permeate thru a1 mil thick membrane composed of polyvinyl chloride at 50° C. The secondside of the membrane is kept at a pressure of less than 0.2 mm. ofmercury. A condenser system is used to collect the permeate at -76° C.Water permeation preference is shown by the precipitation of the maltolfrom the solution as it becomes more concentrated. In general, themembranes of the present invention are useful in removing water from thefollowing amino acids and their derivatives: arginine, histidine,lysine, tyrosine, tryptophan, phenylalanine, cystine, methionine,threonine, serine, leucine, isoleucine, valine, glutamic acid, asparticacid, glycine, alanine, proline, and hydroxyproline.

EXAMPLE 21

A 6% by weight aqueous solution of Bacteria subtilus enzymes at pH 6-7is concentrated to 60% by allowing permeation thru a polyvinyl chloridemembrane at 35° C. The downstream side of the membrane is held at apressure below 0.2 mm. of mercury. The pressure on the liquid side isatmospheric. If this same concentration were achieved via reverseosmosis, the pressure of the feed side would have to be greater than 400psi.

EXAMPLE 22

A liquid containing 5.7 percent by weight of acrylonitrile in water at45° C is kept under a total pressure (by pumping) of 32 psi, andpermeation through a membrane composed of polyvinyl chloride is alowedto occur. The polyurethane block polymer is composed of soft sections ofpolyethylene oxide dialcohol combined by means of hard sections oftoluene disocyanate within the molecular chain. The vapor i.e., permeateside of the membrane is at atmospheric pressure swept by helium to lowerthe chemical potential of the diffusing species. The liquid in the feedcell becomes concentrated in acrylonitrile and a second phase ofpredominantly liquid acrylonitrile appears as the water is permeatedpreferentially into the receiving cell.

EXAMPLE 23

A poly(vinylidene fluoride) film is kept in contact with a 14% by weightsolution of formaldehyde in water at 70° C. The permeate side of themembrane is held at less than 0.1 mm of mercury. The separation factoris 6.

Under the same conditions as in the preceding paragraph a film of poly(vinyl fluoride) give a separation factor of 42.

What is claimed is:
 1. Process for the separation of water from feed mixtures comprising water and formaldehyde which comprises contacting the aforesaid mixture against one side of a polyacetal membrane and withdrawing, at the second side, a vaporous mixture having a higher concentration of water than the aforesaid feed mixture, with the mixture at second side being maintained at a lower chemical potential than the feed side.
 2. Process as in claim 1 in which the feed mixture comprises water, formaldehyde and methanol.
 3. Process as in claim 1 in which the said membrane comprises poly (vinylbutyral).
 4. Process for the separation of water from feed mixture comprising water and formaldehyde which comprises contacting the aforesaid mixture against one side of a polyacetal membrane and withdrawing, at the second side, a vaporous mixture having a higher concentration of water than the aforesaid feed mixture, with the mixture at the second side being maintained at a pressure less than atmospheric, but less than the pressure at the feed side of the membrane. 